This name may be given to that operation of medicines by which they change existing morbid actions or states, without any observable effect on the system to which the result could be ascribed. The medicines are usually called alteratives. They may produce their effects by changing the character of the blood, or the condition of the solids. Their precise mode of action is unknown, or at best conjectural. It will be perceived that the employment of the term is merely a convenient mode of classifying certain unintelligible results, which depend altogether for their acceptance upon the evidence of observation. In certain states of disease we administer certain remedies, without other observable effect than a cure. It is often very difficult, in such cases, to decide whether the result has proceeded from the remedy, or has happened in the ordinary course of the disease. It is at least highly probable that a great many medicines have acquired a credit as alteratives, which was due exclusively to nature.

Among the most striking illustrations of this operation of medicines is that of mercury in the cure of inflammation. After due depletion, or when depletion is not indicated, no remedy has so powerful an antiphlogistic influence as mercury, urged to the point of affecting the system. Other examples are offered by iodine in scrofula, sarsaparilla in venereal disease, colchicum in gout, etc.

It is apparent that supersession and alteration may often lay claim to the same results. Thus, does mercury cure syphilis and inflammation by the substitution of its own transitory morbid effects for the existing disease, or does it merely alter the morbid into healthy action ? Upon the solution of this question it depends, whether the remedy is to be looked upon as a supersedent or an alterative.